Continuous hydrostatic power division transmission

ABSTRACT

A continuous hydrostatic-mechanical power division transmission has at least two operating ranges between which it is switched by shift elements, with a first, mechanical part (II) including a planetary differential gear (13) and a second, hydrostatic part (I) including two power-linked adjustable hydraulic units (H1 and H2) which can be operated in both directions either as a pump or as a motor and which are coupled to the mechanical part (II). An annulus (19) of a planetary differential (13) is used to control the direction and speed of rotation of a gear box output shaft (12) in the individual operating ranges and is coupled to the first hydraulic unit (H1). In a first operating range, the second hydraulic unit (H2) drives via a gear-shift element (K1, K2) to the output shaft (12). The first hydraulic unit (H1) coupled to the annulus (19) works as a pump while the second hydraulic unit (H2) works as a motor. The hydraulic units (H1, H2) are inclined-axis units with respective input and output shafts (27, 41) parallel to each other. Both hydraulic units (H1, H2) are mounted inside the gearbox casing and next to each other so that their respective input and output shafts (27, 41) are oriented in opposite directions and staggered by at most the length of a hydraulic unit with respect to each other in the direction of the axes of symmetry of the input and output shafts; the axes of the cylinders of the hydraulic units are disposed in the same plane; and at least one additional spur pinion stage is fitted between the planetary differential (13) and the second hydraulic unit (H2).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to continuous hydrostatic power divisiontransmissions, and in particular to a hydrostatic transmission part ofsuch a transmission.

2. Description of Related Technology

Continuous hydrostatic power division transmissions, abbreviated hereinas CHP, make it possible to change speed without interruption of tensileforce, with synchronous rpm, and without load. The principle and thestructure of continuous hydrostatic power division transmissions areknown, for example, from the following publications:

1) DE 28 10 086; and

2) DE 29 045 72 C2.

Such transmissions include hydrostatic and mechanical transmissionportions which are coupled to each other in such a way that the power tobe transmitted is transmitted to certain parts in the individual drivingregions hydraulically and/or mechanically. The power division makes itpossible to improve efficiency in comparison to a purely mechanical or apurely hydrostatic transmission. Both hydrostatic units can be driven asa pump and as a motor.

The transmissions that have thus far been realized in the art which arebased on conventional solutions for use in passenger cars and heavyvehicles, have significantly larger dimensions and a correspondinglyincreased weight, and are less cost-effective in comparison to automatictorque converter transmissions.

Based on favorable characteristics, for example, a high start-up torque,better insensitivity of the system, a larger control range in variabledisplacement motors, significantly higher efficiency, as well as thepossibility of higher limiting rpm and speed increasing ratios, the twohydrounits are designed as slanted axis units which can be operated as apump and as a motor. However, the large dimensions and high noise levelthat generally characterize hydrostatic transmissions may beproblematic.

Furthermore, integration of a slanted-axis hydrostat into a CHPtransmission concept has not been possible so far in such a way thatoptimum arrangement of the other components could be achieved with atolerable manufacturing cost. In general, in the conventional solutions,the hydrostatic units are flanged outside of the transmission housing ofthe mechanical transmission part, i.e., all parts that carry highpressure are disposed outside the transmission housing. Optimization ofthe constructional size of the hydrostat has not been possible sinceback gear transmissions have been limited by the distance between theaxles and the shaft diameter. Also, the start-up pulling force was theessential design criterion for the hydrounits to be used. Therealization of a marketable transmission regarding the size of thestructure and its cost has thus so far had limited possibilities.

In a device disclosed in DE 36 24 989 C2 wherein two hydrounits are usedas premounted units with a connecting or control block, the transmissiondesign is very compact.

The hydrostats are disposed in the inner casing of the planetary gearand the control heads of the hydrostats are disposed against each otherat such an angle that their ends almost touch and extend tangentially tothe envelope toward the top and bottom. As a result, the space requiredby the structure is minimized. Disadvantages of such an arrangementconsist above all in increased manufacturing costs for the control blockand realization of energy coupling of the hydrounits based on the angledorientation of the control heads, and thus in providing a flangedesigned for this; furthermore, it is not possible to reduce the size ofthe hydrounits.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more of the problemsdescribed above. It is also an object of the invention to furtherdevelop a CHP unit of the type described in the background sectionherein in such a way that a transmission can be produced which ismarketable from the point of view of size and cost. It is also an objectof the invention that the dimensions and the weight of the transmissioncan be reduced using commercial hydrostatic components to the extentthat these reach the magnitude of transmissions that are on the markettoday, for example, of torque converter transmissions, without having toconsider the technical disadvantages. At the same time, it is ofenormous importance for the use of CHP transmissions in passengervehicles and in the heavy vehicle area to acoustically decouple twohydrounits from the housing in order to reduce the noise level as muchas possible, the noise being caused mainly by the hydrostatic part ofthe transmission.

According to the invention, a continuous hydrostatic-mechanical powerdivision transmission has a mechanical transmission part and ahydrostatic transmission part. The mechanical transmission part includesa planetary differential transmission having at least two sun wheels ofdifferent diameters, a hollow wheel, and a bridge shaft upon whichnonrotatingly connected double planet wheels are disposed, the doubleplanet wheels meshing with the sun wheels, the large sun wheel beingconnected to a transmission input shaft, and the bridge shaft beingconnected to a transmission output shaft.

The hydrostatic transmission part includes first and second displaceablehydrounits energetically coupled with each other, each having adriving/driven shaft and being operable in two directions, as both apump and as a motor. The hydrounits are coupled to the mechanicaltransmission part.

In individual operating regions, the hollow wheel is coupled to thefirst hydrounit to control the direction of rotation and the rate ofrotation of the transmission output shaft. In a first operating range,the second hydrounit is in drive connection with the transmission outputshaft through a change-over element and the first hydrounit which isconnected to the hollow wheel operates as a pump and the secondhydrounit operates as a motor. Furthermore, the hydrounits are disposedin a slanting axis construction and the driving/driven shaft of thehydrounits run parallel to one another. Both hydrounits are disposedwithin a transmission housing. At least one control block is assigned tothe hydrounits to assume valve functions.

According to the invention, the two hydrounits are disposed next to oneanother in such a way that their driving/driven shafts are directedopposite to one another and are displaced with respect to one another inthe direction of an axes of symmetry of the driving/driven shafts of thehydrounits by a maximum amount which corresponds to the length of onehydrounit. The drum axis of each of the hydrounits are disposed in thesame plane. Furthermore, at least one additional spur pinion stage isprovided between the planetary differential transmission and the secondhydrounit.

By disposing two hydrostatic units with their axes slanted and next toone another in such a way that their driving and driven shafts(depending on whether operated as a pump or as a motor) have parallelaxes, but are oppositely oriented and displaced with respect to oneanother a maximum distance of the length of one hydrounit in a directionof the axis of symmetry of the driving shaft or driven shaft of one ofeach of the hydrounits, so that the drum axes are disposed in a plane,there is a possibility of integration of the hydrounits together withthe mechanical transmission part in a common transmission housing,preferably above and below the mechanical transmission part in fittingposition. A sideways arrangement, i.e., one according to the invention,in the vertical direction or fitting position, can also be considered ifthe oil sump is displaced. The hydrostatic transmission part can bedisposed in the region of the mechanical transmission part withoutincreasing the dimensions of the entire transmission with respect to thepurely mechanical part of the transmission. In order to keep thedimensions of the entire assembly as small as possible, the slanted-axishydrostats are preferably oriented with respect to one another in such away that the drums of the hydrounits are directed toward one another ina moved-out state.

The extensions of the axes of symmetry of the driving and driven shaftsof each hydrounit and straight lines drawn through the intersections ofthe outside contours of one of each hydrounit with the symmetry axesdescribe a parallelogram. The energy coupling of the two hydrounits isdone via tubings which are disposed between the two hydrounits in such away that these essentially describe a diagonal of the parallelogram.

The integration of the hydrounits with the mechanical part of thetransmission in a common transmission housing allows creation of atransmission in a compact form into which even the high-pressure partsof the hydrostatic part of the transmission are integrated, so that, incase of leakage, no oil can escape to the outside.

Providing at least one additional intermediate shaft makes it possibleto increase the start-up speed increase ratio of a hydrounit, and thusthe use of smaller hydrostats, as a result of which, in the finalanalysis, the dimensions thereof can be adapted better to the entiretransmission concept. Furthermore, as a result of the speed increaseratios, the same hydrounits can be used in the structure and in thedesign. When using different hydrounits, the jump in structure size canbe reduced considerably.

Preferably, slanted-axis-built hydrounits of the same series can be usedas hydrounits, i.e., the same hydrounits with regard to structure, sizeand design. The placement of the second hydrounit next to the firsthydrounit is accomplished in such a way that an axis is present betweenthe two hydrounits, with reference to which the two hydrounits areoriented with respect to one another with axial symmetry. The positionof the second hydrounit is obtained by rotation of 180° around thisaxis, which is directed perpendicularly to one of the planes defined bythe axis of symmetry of the driving and driven shafts of the hydrounits.The position of the axis is chosen in such a way that the two hydrounitsare disposed directly next to one another without any significantdisplacement in the direction of the axes of symmetry. In such anembodiment, the extension of the axes of symmetry of the driving shaftand driven shaft of each hydrounit and the straight lines laid throughthe intersections of the outside contours of the hydrounits with theextended symmetry axis define a rectangle. The hydraulic connectinglines, preferably designed in the form of tubes, are disposed betweenthe two hydrounits and essentially define a diagonal of the rectangle.In connection with a transverse bar or connecting lines disposed betweenthe two hydrounits, the two hydrounits always describe a letter Z andtherefore this arrangement can also be called a Z arrangement. The twohydrounits are disposed parallel to one another but reversedright-to-left with respect to one another. An especially positive aspectof this embodiment is that this arrangement requires the leastconstructional space in the entire transmission in comparison totransmission arrangements of the conventional design, and thus thepossibility of building a very compact total transmission is created.

Based on the orientation of the driving and driven shafts of theindividual hydrostats and their drum axes in a common plane, noadditional manufacturing expenditure is required in the production ofhydrostats for realization of the energy coupling between thehydrostats, and the same is true for the integration of the controlblock.

The control block is preferably integrated in the connecting block. Withcorresponding disposition of the individual valve functions, the controlblock and connecting block can be structured symmetrically. In addition,there is also the possibility to integrate the adjustment or adjustmentdevices of the hydrounits in the connecting block. This provides theadvantage that the connecting block can be arranged withoutconsideration of the connections or design of the connecting sides ofthe individual hydrounits to the housing end surfaces, that is, theconnecting block can also be incorporated, turned by 180°. Anotherpossibility to make the entire unit significantly more compact consistsin integration of the housing end parts of the hydrounits in theconnecting block.

In the transmission embodiment as disclosed herein, the hydrounits canbe disposed in the region of the outer crown of the planetarydifferential whereby the distance between the axes of the driving anddriven shafts of the two hydrounits depends on the size of the planetarydifferential.

A significant advantage of this embodiment consists furthermore in thefact that the two hydrounits can be combined into a single assembly,which can be premounted, and incorporated into the transmission as asingle assembly. For this purpose, two hydrounits are rigidly joined toeach other with at least one bridge or transverse bridge. The connectionis preferably performed by bridges applied onto the housing. The entireassembly can then be elastically hung in the transmission.

Advantageously, the integration of the hydrostatic part of thetransmission into a transmission housing is accomplished together withthe mechanical part of the transmission. The high-pressure parts are allintegrated in the housing so that in case of leakage, no oil can reachto the outside. The hydrostatic transmission part can be handled andmounted easily in the form of a single assembly. A CHP transmission withtwo hydrounits oriented according to the invention is characterized bysmaller dimensions of the entire transmission, lower noise emission, andlower cost in comparison to CHP units having a conventionally orientedhydrounit.

The magnitude of the distances between the axes of symmetry of thedriving and driven shafts of the hydrounits can be minimized, but isalways dependent on the structure of the mechanical part of thetransmission and on the orientation of the hydrostatic part of thetransmission in the total transmission.

A purely hydrostatic transmission in a very compact form can be createdaccording to an aspect of the invention wherein two hydrostatic unitsare disposed next to one another in such a way that the position of oneof the hydrostatic units is obtained by turning the other hydrostaticunit by 180° around an axis which is perpendicular to a plane goingthrough the axes of symmetry of the driving and driven shafts of thehydrounit, and the displacement of which, in the direction of the axesof symmetry is essentially zero, as well as that the connection of thetwo hydrounits is accomplished with at least one transverse bridge. Thedrum axes of the individual hydrounits are in one plane. The assembly ischaracterized by simple and easy handling and can be obtained completelypremounted and tested.

Other objects and advantages of the invention will be apparent to thoseskilled in the art from the following detailed description taken inconjunction with the drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b shows an arrangement of two hydrounits in one assemblyaccording to the invention.

FIG. 2 illustrates schematically an arrangement of the hydrounits in anarrangement in a CHP transmission according to the invention.

FIGS. 3a to 3c show the structural rearrangement within the totaltransmission concept according to FIG. 1 with a flanged-on controlblock.

FIGS. 4a to 4c illustrate schematically other possibilities of definingthe control block.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1a and 1b shows a preferred embodiment of an arrangement of twohydrostatic units H1 and H2 according to the invention having aslanted-axis structure, and their combination to a single assembly. Thetwo hydrounits H1 and H2, preferably commercially available ones, aredisposed in plane E1, as can be seen from FIG. 1b. The plane E1 isdetermined by the axes of symmetry A1 and A2 of driving and drivenshafts of the individual hydrounits H1 and H2.

Preferably, the two hydrounits are similarly designed. Both hydrounitsare axially symmetrical with respect to an axis A, which is directedperpendicular to the plane E1, that is, the position of the hydrounit H2can be described by turning the hydrounit H1 by 180° around the axis A.The position of the axis A is chosen in such a way that both thehydrounits H1 and H2 are disposed directly next to one another withoutdisplacement in the direction of the axes of symmetry of their drivingand driven shafts. The extensions of these axes of symmetry A1 and A2 ofthe driving and driven shafts of each of the hydrounits H1 and H2 andthe straight lines G1 and G2 drawn through the intersections of theoutside contours of the hydrounits with the axes of symmetry, describe arectangle. Drum axes T1 and T2 of the hydrounits H1 and H2 also lie in acommon plane, which is the same as the plane E1, which goes through theaxes of symmetry A1 and A2 of the driving and driven shafts of the twohydrounits H1 and H2.

The two hydrounits are coupled to each other through a connecting blockincluding connecting lines, in the embodiment shown in the form ofpipelines 3. These are disposed between the two hydrounits H1 and H2.Preferably, the two hydrounits are rigidly joined to a single unit(assembly 8) with at least one cross bridge 7. The cross bridge 7 isjoined to the housings of the two hydrounits. The valve functions areintegrated into a control block 6. In the embodiments shown here, thiscontrol block 6 is disposed on the hydrounit H2 in order to keep thevalves accessible from the outside in case they are contaminated.However, it is possible to integrate the control block 6 directly intothe pipelines 3.

The entire assembly 8 can be connected to the transmission housing, notshown here, via elastic elements 4, and can be coupled through couplings5a and 5b to the corresponding shafts, for example, to the shafts of themechanical transmission part of a CHP transmission.

FIG. 2 shows schematically the arrangement of the two hydrounits in aCHP concept according to an internally made plan. The power divisiontransmission, generally designated 10, consists of a hydrostatictransmission part I, including two hydrounits H1 and H2, which can beoperated as a pump and as a motor, and a mechanical transmission partII. A driving machine, not shown, is connected on the side of thetransmission input shaft 11. A transmission output shaft of the powerdivision transmission 10 is designated 12. There is a planetarydifferential 13 disposed between the transmission input shaft 11 and thetransmission shaft 12. The differential includes a large sun wheel 14, asmall sun wheel 15, double planet wheels 16 and 17, a bridge shaft 18,and a hollow wheel (annulus) 19. The large sun wheel 14 is nonrotatinglyconnected to the transmission input shaft 11. The small sun wheel 15 isnonrotatingly connected to a hollow shaft 20, on which, again, a toothedwheel (gear) 21 is nonrotatingly disposed. The toothed wheel 21 mesheswith a toothed wheel 23 which is disposed on a shaft 22 so that it canrotate. A toothed wheel 39 is nonrotatingly attached to the shaft 22 andmeshes with a toothed wheel 40, which is in nonrotating connection witha shaft 41, which functions as the driving or driven shaft of thehydrounit H2. The 22 represents the mechanical connection between thehydrounit H2 and the planetary differential transmission 13, as anintermediate shaft. Furthermore, a toothed wheel 24 can be nonrotatinglyconnected to the shaft 22 with the aid of a coupling K1, which isengaged with a toothed wheel 25 that is nonrotatingly connected with thetransmission output shaft 12. With the aid of a coupling K2, the toothedwheel 23 can be nonrotatingly coupled to the shaft 22.

A spur pinion SR₁ and a spur pinion SR₂ are nonrotatingly connected withthe hollow wheel 19. The spur pinion SR₁ meshes with acountertransmission VR₁ and the spur pinion SR₂ meshes with acountertransmission VR₂ through a reverse wheel UR. Bothcountertransmissions VR₁ and VR₂, can be coupled optionally withclutches K3 and K4 to a shaft 27, which functions as a driving shaft ora driven shaft of the hydrounit H1.

The hydrounits H1 and H2 are joined together with connecting lines 28and 29. The hydrostatic coupling of the hydrounit H1 to the hydrounit H2is done via a control block H1/H2, which includes two check valves 30and 31 for feeding the amount of oil leaked and two relief valves 32 and33 to limit the maximum pressure.

The oil is supplied to the hydrounits via a feed pump 37 driven by thedriving machine through a spur pinion stage 34, including a spur pinion35, which is nonrotatingly joined to the transmission input shaft 11,this spur pinion being engaged to a spur pinion 38 which isnonrotatingly joined to the drive shaft 36 of the feed pump 37.

The rate of rotation of the transmission output shaft 12 is the sum ofthe rates of rotation of the large sun wheel 14 and of the hollow wheel19, which determine the of rotation of the planet wheels 16 and 17 andof bridge shaft 18.

In a top view, the two hydrounits H1 and H2 are arranged in such a waythat their driving and driven shafts 41 and 27 are directed opposite toone another, as a result of which, in contrast to the internal solution,the additional intermediate states 39/40 could be integrated. Theadvantage of this is that the speed increase ratio to hydrounit H2 canbe increased by about 50% without exceeding the limiting rates ofrotation of the hydrostats.

FIGS. 3a-c show a constructive arrangement of the two hydrounits intoone assembly 8 according to FIG. 1 in a total transmission concept of acontinuous hydrostatic power division transmission according to FIG. 2.Therefore, in the following description, the same reference numbers areused for the same elements.

FIG. 3a shows the arrangement of assembly 8 which was already describedin detail in FIG. 1, in a total transmission concept according to FIG. 2in a top view. However, the mechanical part of the transmission is notshown for the sake of clarity. The hydrounits H1 and H2, which arecombined to an assembly 8 with the aid of the transverse bridge 7, areconnected to the transmission housing 9 with the aid of the elasticelements 4a and 4b. The mechanical transmission part 1, which is notshown here for the sake of clarity, is also integrated into thistransmission housing 9. The hydrounits H1 and H2 are coupled to shafts27 and 41 through couplings 5a and 5b, which can be designed, forexample, in the form of a curved teeth couplings. The hydrounits arecoupled to the planetary differential transmission through spur pinionsnonrotatingly connected to these shafts, but the spur pinions are notshown here individually.

FIG. 3b shows the top view as shown in FIG. 3a, with the mechanicaltransmission part drawn in. The hydrounit H2 is disposed below theintermediate shaft 22.

FIG. 3c shows a section through the transmission according to FIG. 3b.It can be seen from this representation that, when the hydrounits are ofa small size, there is a possibility to dispose the hydrounitsessentially below the outer crown of the planetary differentialtransmission 13. However, here, only the head circle diameters of theindividual elements of the planetary differential transmission areshown. In the embodiment that is illustrated here, the arrangement isaccomplished in connection with the arrangement shown in FIG. 3b belowthe hollow wheel 19 and above the spur pinion 25 connected to bridgeshaft 18. This has the advantage that the space required by themechanical part of the drive does not have to be enlarged in the axialdirection as it would be in any embodiment in which the mechanical andhydrostatic transmission parts would be displaced in the axialdirection. Both of the hydrounits are disposed in the transmission sump.

FIG. 4a shows schematically, and as an example, the integration of thecontrol block with two pressure limiting valves into the connectingblock 3. Preferably, the control block is designed in such a way that apressure limit valve is assigned to each hydrounit H1 and H2 on thehigh-pressure side. This provides the advantage that the control block6, and thus the connecting block 3, can be constructed symmetrically, asa result of which, the control can be disposed on the housing endsurfaces of the individual hydrounits without consideration of theconnections or the design of the connecting sides.

FIG. 4b shows the additional integration of hydrostat adjustments (40,41) in the connecting block 3, by way of example.

FIG. 4c schematically shows the constructive design in a very simplifiedform. The connecting block 3 can be flanged onto the end housing of theparticular hydrounit H1 and H2. However, it is also possible tointegrate the end housing of the hydrostats with the connecting block.

According to another aspect of the invention, the hydrostatictransmissions can also be designed in the form of the assembly describedhere when the driving and driven shafts are not very far-removed fromeach other.

I claim:
 1. In a continuous hydrostatic-mechanical power divisiontransmission having a mechanical transmission part and a hydrostatictransmission part and wherein(a) the mechanical transmission partcomprises a planetary differential transmission having at least two sunwheels of different diameters, a hollow wheel and a bridge shaft uponwhich nonrotatingly connected double planet wheels are disposed, saiddouble planet wheels meshing with said sun wheels, the large sun wheelbeing connected to a transmission input shaft and the bridge shaft beingconnected to a transmission output shaft; (b) the hydrostatictransmission part comprising first and second displaceable hydrounitsenergetically coupled with each other, each having a driving/drivenshaft, each of said hydrounits operable in two directions as a pump andas a motor, said hydrounits being coupled to the mechanical transmissionpart; (c) in individual operating regions, the hollow wheel is coupledto the first hydrounit to control the direction of rotation and the rateof rotation of the transmission output shaft; (d) in a first operatingrange, the second hydrounit is in drive connection with the transmissionoutput shaft through a change-over element and the first hydrounit whichis connected to the hollow wheel operates as a pump and the secondhydrounit operates as a motor; (e) the hydrounits are disposed in aslanting axis construction; (f) the driving/driven shaft of thehydrounits run parallel to one another.
 2. The improvement of claim 1wherein:(a) the hydrounits have the same design and constructional sizeand are defined by the following characteristics: (b) the position ofthe second hydrounit can be defined by rotating the first hydrounitaround an axis A by 180°; and (c) a position of the axis A is chosen insuch a way that displacement between the two hydrounits in the directionof the axes of symmetry is almost zero.
 3. The improvement of claim 1wherein(a) the hydrounits have extended housing end parts; and (b) eachend part is integrated in a connecting block.
 4. The improvement ofclaim 1 wherein a control block is integrated into a connecting blockbetween the two hydrounits.
 5. The improvement of claim 4 wherein(a) thecontrol block includes at least first and second pressure controlvalves; and (b) one of said pressure control valves is assigned to eachhydrounit on a high-pressure side thereof.
 6. The improvement of claim 4wherein the connecting block is substantially symmetrical.
 7. Theimprovement of claim 1 further comprising displacing devices of thehydrounits being integrated in the connecting block.
 8. The improvementof claim 4 wherein the control block is flanged on a housing of at leastone of the hydrounits.
 9. The improvement of claim 1 wherein the twohydrounits fit in a position in a region of an outer crown of theplanetary differential transmission.
 10. The improvement of claim 1wherein the two hydrounits are rigidly connected to each other forming astructural unit.
 11. The improvement of claim 10 wherein the connectionof the hydrounits is provided by at least one transverse bridge which islinked to a housing of each hydrounit.
 12. The improvement of claim 10wherein the structural unit is elastically hung in a transmissionhousing.
 13. The improvement of claim 1 wherein(a) in the firstoperating range and in a second operating range , the two hydrounits runopposite to one another in a range between minimum and maximum, and whenchanging from the first to the second operating range and vice versa,the hydrounits exchange function; (b) the hollow wheel can be coupledwith the driving/driven shaft of the first hydrounit with the aid of asecond change-over element optionally through at least one of a firsttoothed wheel drive and a second toothed wheel drive; (c) the firsttoothed wheel drive being built for coupling to the hollow wheel to thefirst hydrounit through the first toothed wheel drive, the hollow wheeland the driving/driven shaft of the first hydrounit rotate in theopposite direction; and (d) the second toothed wheel drive is built forcoupling with the first hydrounit, the hollow wheel and the driving ordriven shaft of the hydrounit rotate in the same direction.